I believe that what you're seeing is the result of the video signal being higher than the monitor can deal with, and the interface between edges is "vibrating" at the pixel level - especially if this is interlaced footage. BTW, hope you used the scopes and 3-way color corrector to fix it, because the broadcast safe filter is just a quick fix and not always actually within the strictest of broadcast tolerances. People too often use it quickly and unthinkingly as a crutch and only later does it bite them. The broadcast safe filter basically clamps the brights and darks but you might lose a lot of nuance in what's in-between so it's just better over-all practice to color correct with the 3-way.

Now I don't know why you're getting footage that is so very far out of spec to begin with, but you may want to go thru your overall signal chain/ intake practices and see if the footage was indeed that over-expsoed to start with (then dope slap the shooter) or do you have an analogue stage in your signal flow that perhaps has an unterminated signal? That can sometimes lead to overly-bright signals. When you get a chance, do an end-to-end check of your setup using color bars and scopes and see thateach stage is as close to spec as possible, so your monitors really show you what you're doing.

That sky is blown out: all the detail was lost when you went over 100 IRE units. This is a good candidate for a sky replacement done in compositing. Add back a blue sky gradient using the blown out white as a luminance key, engage motion tracking, to lock the sky to the land, and it will look better.

The footage is 1080p, and it's not just this shot, it is any where the highlight values exceed 100.
The dashed lines do not show up on QT export they are definitely the result of an overlay or display issue within FCP.

As for sky replacement/color correction the online will deal with any actual color correction, I just used that shot as a simple easy to see example of the issue.

I just need to work out what is generating the overlay dashed lines and turn it off, before it drives all the offline editors crazy.

Bottom line is that it isn't an overlay. It's just that FCP will show you the where the whites are blown out. It's so blown out in those areas that there's no info. So it displays it as black. Not an overlay. I doubt you can fix that with standard filters in FCP. Sky replacement is a good option. But not perfect. Know a really good After Effects artist? Rotobrush might be workable in AE for rotoscoping that.